1. Field of the Invention
This invention relates to optical transmission systems and more specifically to a fiber optic transmission system capable of carrying a broadband signal over single mode optical fiber with high optical launch power.
2. Technical Background
Long haul submarine optical fiber cable systems having optical cable spans underwater are typically deployed between continents and or along the coastline of continents. Repeatered submarine systems employ repeaters along their length. A repeater is a unit typically in the form of an enclosed box, which contains an amplifier to boost signal strength and an equalizer to correct distortion. Typically such repeaters are placed at intervals along the submarine optical cable to allow longer cables to be used and/or to increase system reach. Typically, repeaters contain at least one electrical component.
As used herein, “active repeater” refers to a repeater having at least one electrical component, and “unrepeatered” or “repeaterless” refers to the absence of active repeaters.
Unrepeatered systems typically do not extend further than about 500 kilometers, and in most instances are considerably shorter than 500 kilometers. Unrepeatered systems are very often used in festoon applications, wherein the optical cable is disposed between a transmitter and receiver at different locations, for example along a coastline with the festoon optical fiber being deployed under water between the transmitter and receiver.
Unrepeatered systems play a valuable role in linking markets, particularly markets separated by distances of 100 to 500 km. For example, festoon systems comprised of undersea networks are used to connect islands or to circumvent difficult geological impediments or unstable political routes. Several unrepeatered systems have been installed on long overland routes that do not require add/drop capabilities but could use branching units. Such applications offer a method of transmitting optical signals over several hundred kilometers.
Fiber attenuation losses in an unrepeatered system can be on the order of, for example, approximately 20-60 dB when span lengths of 100-300 km are employed for a fiber having an attenuation of 0.20 dB/km. Signal to noise constraints generally constrain system lengths to less than about 220 km when the sole source of amplification is an EDFA at the transmitter. The addition of distributed Raman amplification may increase the maximum length by 50 to 130 km. Raman pump lasers are usually backward-propagating from the receiver end, but forward pumping may also be employed. Distances of 350-500 km can be achieved through the use of one or more Remote Optically Pumped Amplifiers (ROPAs), which consist of a length of Erbium-doped fiber which is spliced into the transmission path and pumped using the same fiber or an additional length of fiber that is optically coupled to the transmission fiber near the Erbium-doped section.
The most commonly used fiber in unrepeatered systems has been conventional single mode fiber because of its combination of low attenuation, effective area of about 80 μm2 at 1550 nm, low price and wide availability. Such standard singlemode fiber may be comprised of a germania doped silica fiber, such as Corning SMF-28™ fiber, or pure silica core fiber. More recently, large effective area cutoff-shifted fibers such as Corning Vascade® L1000 have become available, which offer an effective area of about 101 μm2 at 1550 nm.
The need to increase the channel count per fiber requires higher power handling capacity. Increasing the maximum distance, or reach, requires higher input powers, which in turn increases single channel nonlinearities such as self-phase-modulation (SPM) and Stimulated Brillouin scattering (SBS). For a given optical fiber effective area, decreasing the channel spacing results in increased penalties from inter-channel effects such as cross-phase modulation (XPM) and four-wave-mixing (FWM), particularly at bit rates of 10 Gb/s and higher. An increase in the fiber effective area reduces both single-channel and inter-channel nonlinearities.
The transmission of digital signals over long unrepeatered distances requires the use of high power optical amplifiers, therefore giving rise to concerns about SBS suppression and self-phase modulation (SPM). See Y. Aoki et al., “Non-repeatered long distance transmission systems with remote pumping and WDM technologies,” pp. 555-563, Proceedings of 1997 SubOptic Conference. Stimulated Brillouin scattering (SBS) is a nonlinear optical effect that poses a significant restriction to the amount of narrow-linewidth optical power that can be launched into a long length of single mode optical fiber. For a given length of single-mode fiber with a given attenuation coefficient at the chosen optical wavelength, there is an optical-linewidth-dependent threshold power below which SBS does not occur. For standard commercially available telecommunication fiber operating at 1550 nm, the SBS threshold for a continuous wave (cw) optical source (laser) with an optical linewidth less than 10 MHz is less than 7 dBm for a fiber optic link of approximately 50 kilometer length.
SBS suppression is as typically achieved in known systems by broadening the laser linewidth or artificially spreading the signal spectrum by external phase modulation. However, broadening the signal linewidth can impact system performance by increasing the overlap of signal pulses in a single channel (SPM) or adjacent channels (XPM), while phase modulation of the signal spectrum can be converted to intensity modulation and signal distortion by nonlinearities in the system. SBS suppression in known systems appears to limit the average signal power per channel and the span length due to nonlinearities.
The maximum average channel power for known optical fiber has been mapped out as a function of transmission distance for 100 km lengths of single mode and non-zero dispersion-shifted fibers. See G. Mohs et al., “Maximum Link Length versus Data Rate for SPM Limited Transmission Systems,” ECOC 2000. The SPM limit for the known fibers was found to be greater than 18 dBm for bit rates of 10 and 20 Gb/s and for various modulation formats. At 40 Gb/s, the SPM limit was greater than 15 dB for 35% RZ modulation, very near the 33% RZ used in most known unrepeatered transmission systems. System performance typically drops in known systems for average channel powers greater than about 18 dBm due to SPM and inter-channel nonlinearities. See N. H. Taylor and W. D. Cornwell, “The Use of High Effective Area Fibre in Submarine Festoon Systems,” pp. 333-336, Proceedings of SubOptic 2001.